Plant maintenance method and apparatus

ABSTRACT

An apparatus for maintaining plant equipment incorporates an input unit to receive information on targets and ranges of plant equipment to be maintained and a failure-event tree breakdown unit to perform tree-breakdown on failures to be expected from events to be inspected. A failure-unreliability function calculation unit calculates unreliability of each item in failure-event tree breakdown and a failure-derived monetary loss calculation unit multiplies each unreliability and a cost of recovery from each event in accordance with the failure-event tree breakdown. A preventive maintenance expense calculation unit calculates preventive maintenance expenses for preventing failure events and a maintenance decision-making unit decides maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expense.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-106666, filed on Apr. 9,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to plant-maintenance apparatus and methodfor maintaining plant equipment such as steam turbines for thermal powerplants.

Increase in the number of plants such as thermal power plants used forlong years has demanded maintenance at low cost but with no failures.Risk-based maintenance (RBM) has been recently introduced as one of thesolutions to match the demands. A known RBM is to decide maintenancepriority allocations based ranking in the order of combinations ofclassified plant-equipment failure rates and consequence of failure,such as, disclosed by Kihara et el., in Piping Technology, pages 76 to79, issued in December, 2000.

Classification of consequence of failure, however, requires experimentalqualitative decision making, and hence has several drawbacks, forexample, it does not necessarily involve quantitative evaluation.

SUMMARY OF THE INVENTION

With the foregoing as background, it is an object of the presentinvention to provide a method and an apparatus for objectively andquantitatively deciding the optimum maintenance timing by risk-basedcost estimation with extracted information on plant-equipment failuresand expected events due to the failures or on-line monitoring of theleast number of failure-related inspection items.

To achieve the objectives, according the first aspect of the presentinvention, an apparatus for maintaining plant equipment is provided, theapparatus including: an input unit configured to receive information ontargets and ranges of plant equipment to be maintained, that constitutea plant, inspection information and operation-history information; afailure-event tree breakdown unit configured to perform tree-breakdownon failures to be expected from a failure-events to be inspected, inorder to obtain a failure-event tree; a failure-unreliability functioncalculation unit configured to calculate unreliability of a resultantevent in the failure-event tree with respect to a causal event in thefailure-event tree preceding the resultant event (i.e., unreliabilitybetween each item in the failure-event tree); a failure-derived monetaryloss calculation unit configured to multiply each unreliability and acost of recovery from each event to be inspected and accumulate resultsof multiplication in accordance with the failure-event tree breakdown,thus obtaining a recovery cost; a preventive-maintenance expensecalculation unit configured to calculate preventive maintenance expensesfor preventing failure events; and a maintenance decision-making unitconfigured to decide maintenance timing and technique through comparisonbetween the recovery cost and the preventive maintenance expenses.

The present invention further provides an apparatus for maintainingplant equipment, which includes: an input unit configured to receiveinformation on targets and ranges of plant equipment to be maintained,that constitute a plant, inspection information and operation-historyinformation; an inspection-priority item selection unit configured toselect a priority item, which relates to a failure event in afailure-event tree and is stored in a failure-event tree database, asitems to be inspected; an inspection unit configured to inspect theselected items; a failure-event tree breakdown unit configured toperform tree-breakdown on failures to be expected from a failure-eventsto be inspected, in order to obtain a failure-event tree; afailure-unreliability function calculation unit configured to calculateunreliability of a resultant event in the failure-event tree withrespect to a causal event in the failure-event tree preceding theresultant event; a failure-derived monetary loss calculation unitconfigured to multiply each unreliability and a cost of recovery fromeach event to be inspected and accumulate results of multiplication inaccordance with the failure-event tree breakdown, thus obtaining arecovery cost; a preventive-maintenance expense calculation unitconfigured to calculate preventive maintenance expenses for preventingfailure events; and a maintenance decision-making unit configured todecide maintenance timing and technique through comparison between therecovery cost and the preventive maintenance expenses.

The present invention further provides an apparatus for maintainingplant equipment, which includes: an input unit configured to receiveinformation on targets and ranges of plant equipment to be maintained,that constitute a plant, inspection information and operation-historyinformation; a monitoring-item selection unit configured to select apriority item, which relates to a failure event in a failure-event treeand is stored in a failure-event tree database, as items to bemonitored; an on-line monitoring unit configured to monitor the selecteditems; a failure-event tree breakdown unit configured to performtree-breakdown on failures to be expected from a failure-events to beinspected, in order to obtain a failure-event tree; afailure-unreliability function calculation unit configured to calculateunreliability of a resultant event in the failure-event tree withrespect to a causal event in the failure-event tree preceding theresultant event; a failure-derived monetary loss calculation unitconfigured to multiply each unreliability and a cost of recovery fromeach event to be inspected and accumulate results of multiplication inaccordance with the failure-event tree breakdown, thus obtaining arecovery cost; a preventive-maintenance expense calculation unitconfigured to calculate preventive maintenance expenses for preventingfailure events; and a maintenance decision-making unit configured todecide maintenance timing and technique through comparison between therecovery cost and the preventive maintenance expenses.

The failure-event tree breakdown unit may be provided with afailure-event tree database constructed based on inspection historiesand operation histories for the plant to be maintained and other plants;and the failure-event tree breakdown unit may be configured to performthe tree-breakdown from a specific failure-event of a specific part ofthe plant to be maintained inputted by an input unit, while referring tocontents of the database.

The failure-unreliability function calculation unit may be provided witha failure-unreliability function database constructed based oninspection histories and operation histories for the plant to bemaintained and other plants; and the failure-unreliability functioncalculation unit may be configured to calculate the unreliability whilereferring to the database based on failure-unreliability functionbetween the resultant event and the causal event in the failure-eventtree in accordance with information on parts of the plant and eventsinputted via the input unit.

The failure-event tree breakdown unit and the failure-unreliabilityfunction calculation unit may be configured to adjust the order of thefailure events in the failure-event tree and failure-unreliabilityfunction.

The unreliability of the plant equipment may be given using cumulativehazard function based on an operation time at which each failure eventoccurs.

According to the second aspect of the present invention, a method ofmaintaining plant equipment is provided, the method including the stepsof: receiving information on targets and ranges of plant equipment to bemaintained, that constitute a plant, inspection information andoperation-history information; performing tree-breakdown on failures tobe expected from events to be inspected, thereby obtaining afailure-event tree; calculating unreliability of every resultant eventin the failure-event tree with respect to corresponding causal event inthe failure event tree; multiplying each unreliability and a cost ofrecovery from each event to be inspected and accumulating results ofmultiplication in accordance with the failure-event tree breakdown,thereby obtaining a recovery cost; calculating preventive maintenanceexpenses for preventing failure events; and deciding maintenance timingand technique through comparison between the recovery cost and thepreventive maintenance expenses.

The present invention further provides a method of maintaining plantequipment, which includes the steps of: receiving information on plantequipment to be maintained, that constitute a plant, inspectioninformation and operation-history information; selecting a priorityitem, which relates to a event in a failure-event tree and is stored ina failure-event tree database, as items to be inspected; inspecting theselected item; performing tree-breakdown on failures to be expected fromevents to be inspected, thereby obtaining a failure-event tree;calculating unreliability of every resultant event in the failure-eventtree with respect to corresponding causal event in the failure eventtree; multiplying each unreliability and a cost of recovery from eachevent to be inspected and accumulating results of multiplication inaccordance with the failure-event tree breakdown, thereby obtaining arecovery cost; calculating preventive maintenance expenses forpreventing failure events; and deciding maintenance timing and techniquethrough comparison between the recovery cost and the preventivemaintenance expenses.

The present invention further provides a method of maintaining plantequipment, which includes the steps of: receiving information on targetsand ranges of plant equipment to be maintained, that constitute a plant,inspection information and operation-history information; selecting apriority item, which relates to a event in a failure-event tree and isstored in a failure-event tree database, as items to be monitored;monitoring the selected items; performing tree-breakdown on failures tobe expected from events to be inspected; calculating unreliability ofeach item in the failure-event tree breakdown; multiplying eachunreliability and a cost of recovery from each event to be inspected andaccumulate results of multiplication in accordance with thefailure-event tree breakdown, thus obtaining a recovery cost;calculating preventive maintenance expenses for preventing failureevents; and deciding maintenance timing and technique through comparisonbetween the recovery cost and the preventive maintenance expenses.

The present invention further provides a method of maintaining plantequipment, which includes the steps of: transmitting, over a network,data from inspection and monitoring units for equipment that constituteeach of a plurality of plants, each plant having a plurality of units;processing the transmitted data through failure-event tree breakdown,failure-unreliability function calculation, failure-derived monetaryloss calculation, failure-derived monetary loss calculation andpreventive-maintenance expense calculation; and storing the processeddata in a failure-event tree database and a failure-unreliabilityfunction database for learning, for accurate failure-unreliabilityfunction calculation in maintenance decision making; and transmittingresults of maintenance decision making to the plants.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a block diagram of a first embodiment according to thepresent invention;

FIG. 2 shows a block diagram indicating failure-event tree breakdown inthe first embodiment according to the present invention;

FIG. 3 shows a matrix indicating a failure-unreliability functiondatabase in the first embodiment according to the present invention;

FIG. 4 illustrates unreliability functions in the first embodimentaccording to the present invention;

FIG. 5 shows a list of recovery cost in the first embodiment accordingto the present invention;

FIG. 6 illustrates comparison between a recovery-cost function and apreventive-maintenance expense function;

FIG. 7 shows a block diagram of a second embodiment according to thepresent invention;

FIG. 8 shows a block diagram indicating failure-event tree breakdown inthe second embodiment according to the present invention;

FIG. 9 illustrates a nozzle-erosion inspection unit in the secondembodiment according to the present invention;

FIG. 10 shows a matrix indicating a failure-unreliability functiondatabase in the second embodiment according to the present invention;

FIG. 11 shows a list of recovery cost in the second embodiment accordingto the present invention;

FIG. 12 illustrates measurements of cracks caused in valves;

FIG. 13 shows a block diagram of a third embodiment according to thepresent invention;

FIG. 14 shows a block diagram indicating failure-event tree breakdown inthe third embodiment according to the present invention;

FIG. 15 shows a matrix indicating a failure-unreliability functiondatabase in the third embodiment according to the present invention; and

FIG. 16 shows a block diagram of a fourth embodiment according to thepresent invention;

DETAILED DESCRIPTION OF THE EMBODIMENTS

Several embodiments according to the present invention will be disclosedwith reference to the attached drawings.

FIG. 1 shows a block diagram of a first embodiment according to thepresent invention. A plant-maintenance apparatus incorporates an inputunit 1, a failure-event tree breakdown (FETB) unit 2, afailure-unreliability function calculation (FUFC) unit 3, afailure-derived monetary loss calculation (FDMLC) unit 4, apreventive-maintenance expense calculation (PMEC) unit 5 and amaintenance decision-making (MDM) unit 6.

Entered via the input unit 1 are targets and ranges or a list of plantcomponent parts to be maintained, an operation history such as operationhours and operation-startup/shutdown times and an inspection historyincluding inspection data on failures, damage, degradations and theirindications. Disclosed below are operations of the first embodiment forinspection of falling-down of a nozzle diaphragm of a steam-turbine.

A steam-turbine nozzle diaphragm could suffer from creep deformation dueto steam-pressure differences and hence fall down behind (towarddownstream side) and finally touch a rotor or moving-blades, thus beingdamaged. In detail, several failure events can be expected from, forexample, the amount of deformation of the nozzle diaphragm as inspectioninformation. Those expected failure events are as follows. The nozzlecontacts to the rotor or the moving-blades due to deformation of thenozzle diaphragm. This result in widening of the steam-passage gaps,such as gaps between the tips of the moving-blade and the opposingmember. This results in increase in leakage-loss and thus reduction ininternal efficiency. The contact of the nozzle with the moving-blades orthe rotor wheel also causes cracks in the moving-blades or the rotor dueto rubbing or vibration. This results in scattering of cracked portionsof the moving-blades, and due to this, the nozzle is broken. The contactof the nozzle with the moving-blades or the rotor wheel also causescracks in the rotor wheel and failure of the rotor wheel. The FETB unit2 constructs a failure-event tree indicating causal sequences, such asshown in FIG. 2, based on data stored in a failure-event tree database 7on the basis of targets and ranges to be maintained, an operationhistory, an inspection history, etc.

The FUFC unit 3 extracts the causal sequences from the FETB unit 2 toconstruct an interrelated matrix, such as shown in FIG. 3, and gives thematrix items unreliability F(t)=1−R(t) in which R(t) indicatesreliability.

The unreliability F(t) shows the rate of failures occurring up to agiven time “t”, which can be calculated by using the cumulative hazardfunction, as follows:

The reliability R(t) having the relationship F(t)=1−R(t) with theunreliability F(t) has the following relationship with the cumulativehazard function H(t).R(t)=exp(−H(t))  (Expression 1)

The cumulative hazard function is obtained by accumulation of failurefunction λ(χ), indicating failure probability per unit of time from time“0” to time “t” and expressed as follows: $\begin{matrix}{{H(t)} = {\int_{0}^{t}{{\lambda(x)}\quad{\mathbb{d}x}}}} & \left( {{Expression}\quad 2} \right)\end{matrix}$

An assessed value H′(t) is then given for the cumulative hazardfunction, as follows:

An assessed value H′(t) of the cumulative hazard function H(t) at timet_(k) is given by the following expression (3) for the “n” number ofobserved values t1≦t2 . . . ≦tnH′(t)=Σ1/(n+1−i)  (3)where “i” is “1” to “k”.

It is expressed as H(t)=(t/η)^(m) (“η” and “m” being constants) when theWeibull cumulative hazard function H(t) is used.

Accordingly, the unreliability F(t) is given from the reliability R(t)that is given by the expression (1), using the assessed value H′(t) ofthe cumulative hazard function. Illustrated in FIG. 4 is theunreliability F(t) that becomes higher as the operation time elapses. Asdiscussed, the unreliability F12, F13, . . . are calculated for thematrix items in the interrelated matrix shown in FIG. 3, and areassigned to the corresponding resultant events.

In the failure-event breakdown process, anterior-event reliabilities areallocated on the diagonal line in FIG. 3, with the unreliabilitycalculated from the operation startup. Events occurring after aparticular event are assigned to the items shown in FIG. 3, withcalculated posterior-event unreliability.

FIG. 4 teaches that the posterior-event unreliability rapidly increasesas indicated by a dot line compared to the anterior-event unreliabilityindicated by a solid line.

Then, the FDMLC unit 4 calculates the total risk cost Cr with thefollowing expression using unreliability Fij and a recovery cost Ci(failure-derived monetary loss) pre-assigned to each event.Cr=Σ( . . . Fhi·Fij)Ci

Posterior probability can be employed as the unreliability Fij. Thisrequires multiplication of the posterior probability by unreliability ofan event occurring before the event in question. Alternatively,unreliability combined with unreliability of an event occurring beforethe event in question may be assigned to each item in FIG. 3.

The PMEC unit 5 calculates preventive maintenance expenses, as indicatedby a dot line in FIG. 6, with addition of depreciation and severalmaintenance fees to preventive maintenance expenses. The preventivemaintenance expenses are sent to the MDM unit 6 and expressed as atime-based function and compared with a function of the total risk costCr calculated by the FDMLC unit 4. It is then decided to performmaintenance when the former function is lower than the latter function.

The breakdown order of the items (i.e., events) in the failure-eventtree and the unreliability are adjusted by the FETB and FUFC units 2 and3 in accordance the contents of the plant to be maintained andoccurrence of plant failures stored in the failure-event tree database 7and also a failure-unreliability function database 8 shown in FIG. 1.

In addition to the inspection histories (failure histories), an originalform of the failure-event tree is stored beforehand in the failure-eventtree database 7. The original form of the failure-event tree is made byexpert engineers of the plant engineering by analyzing inspectionhistories and failure-events of plants similar to the plant to bemaintained and by examining the correlation between the failure-eventsbased on their technical experience. When the failure-event treedatabase 7 receives new inspection data on the plant to be maintainedand/or on other similar plants, the FUFC unit 3 re-calculates theunreliability between the failure events. The FETB 2 unit modifies theoriginal form of the failure-event tree (or the failure-event treehaving been previously modified) stored in the failure-event treedatabase 7 on the basis of the re-calculation of the unreliability, inother words, the FETB unit 2 performs re-breakdown of the failure-eventtree. In detail, in the event that the unreliability of “nozzle failure”with respect to “moving-blade crack” becomes higher than theunreliability of “moving-blade scattering” with respect to “moving-bladecrack” as a result of the renewal of the data stored in thefailure-event tree database 7, the FETB unit 2 reverses the order of“nozzle failure” and “moving-blade scattering” in the failure-event treeshown in FIG. 2. In addition, in the event that “nozzle failure” isoccurred after the occurrence of “nozzle diaphragm creep deformation”without occurring “contact between nozzle and moving-blade or rotorwheel”, “moving-blade crack”, and “moving-blade scattering”, the FETBunit 2 makes a new branch directly connecting “nozzle diaphragm creepdeformation” to “nozzle failure” in the failure-event tree shown in FIG.2.

As mentioned above, risks are expressed as costs against any failureevents that could happen to plant component parts and compared with thepreventive maintenance expenses, thus the first embodiment offering alowest-cost maintenance technique with the lowest risk possibility.

Moreover, breakdown of the failure-event tree in accordance with data onequipment parts and events entered based on the failure-event treedatabase constructed based on plant equipment and plant operation andinspection histories, and also unreliability calculation based onfailure functions between failure events in the broken-down tree cancover all items to be subjected to risk calculation and offerreliability evaluation that matches the actual plant conditions.

Furthermore, adjustments of the breakdown order of the items (i.e.,failure events) in the failure-event tree and the adjustments of theunreliability, which are performed by the FETB and FUFC units 2 and 3 inaccordance with the entry of data on the plant component parts and plantoperation and inspection histories, offer further accurate riskevaluation according to the stored data.

The unreliability may be given to any events depending on the number ofstart-up by using the cumulative hazard function from the number ofstart-up at different operating times.

FIG. 7 shows a block diagram of the second embodiment of the maintenanceapparatus according to the present invention. Elements shown in FIG. 7the same as or analogous to elements shown in FIG. 1 are referenced bythe same numerals.

With this embodiment, the apparatus further provided with aninspection-priority item selection (IPIS) unit 9 configured to select aninspection item. Preferably, the IPIS unit 9 selects an inspection itemthat can be inspected without disassembling the equipment (e.g.,turbine) based on data on targets and ranges to be maintained enteredvia the input unit 1. In this embodiment, the IPIS unit 9 selects theerosion of the first-stage nozzle blade as the inspection item, which isthe summit event in the failure-event tree.

As shown in the failure-event tree of FIG. 8 in which the nozzle-bladeerosion is the summit event, a strainer or other parts such as valveslocated upstream of the first-stage nozzle erode when the first-stagenozzle blade erodes (with a little time difference). The advancement ofnozzle-blade erosion leads to change in area of the nozzle throat thatcauses change in steam flow, thus resulting in reduction in the internalefficiency. This induces fatigue due to erosion of moving-blades,shrouds and tenons or vibration of the moving-blades, thus causingbreakage of the moving-blades located downstream of the nozzle, and thenthe rotor. These breakages lead to breakage to the first-stage nozzle.

As shown in FIG. 9, the second embodiment employs a nozzle-erosioninspection unit 10 that is a remote image-processing unit 11 equippedwith a CCD-built-in fiber scope and an optical guide. The fiber scope isinserted into the steam pipe to take pictures of the first-stage nozzleblade for image processing in erosion measurements. The image taken bythe fiber scope to be processed may be a still image or a moving image.

Like the first embodiment, the FETB unit 2 constructs a failure-eventtree indicating causal sequences, such as shown in FIG. 8, based on datastored in the failure-event tree database 7 in accordance with targetsand ranges to be maintained, an operation history, an inspectionhistory, etc.

The FUFC unit 3 extracts the causal sequences from the FETB unit 2 toconstruct an interrelated matrix, such as shown in FIG. 10, and givesthe matrix items unreliability F(t)=1−R(t) in which R(t) indicatesreliability.

Change in amount of erosion is expressed as time-based function from thedetachment of boiler-scale or incrustation. It is, however, difficult topredict when the boiler-scale detaches. Thus, it is statistically decidewhen the amount of erosion becomes constant, with tendency analysis ofinspected data, which is then used for unreliability calculation.

Then, the FDMLC unit 4 calculates the accumulated total risk cost Crusing unreliability Fij and a recovery cost Ci pre-assigned to eachevent, as shown in FIG. 11. The PMEC unit 5 calculates the totalpreventive maintenance expenses.

Like the first embodiment, depreciation and several maintenance fees areadded to the preventive maintenance expenses. The expenses are sent tothe MDM unit 6 and expressed as a time-based function and compared witha function of the total risk cost Cr calculated by the FDMLC unit 4. Itis then decided to perform maintenance when the former function is lowerthan the latter function.

Illustrated in FIG. 12 is inspection of cracks caused in valves with anultrasonic inspection sensor 12. The failure-event tree breakdown,failure-unreliability calculation and cost evaluation disclosed abovecan be applied to these cracks.

As disclosed in detail, risks can be accurately assessed with theminimum number of inspection items gained for the least plant-haltperiod without equipment disassembly, thus the second embodimentoffering a maintenance method at the lowest inspection and diagnosiscosts.

Moreover, inspection of internal damage to plant equipment with the CCDcamera or the ultrasonic inspection sensor (the images being sent asnumerical data to an image processor) quickly offers inspection resultson the inspection-priority items in the failure-event tree breakdown.

FIG. 13 shows a block diagram of the third embodiment of the maintenanceapparatus according to the present invention. Elements shown in FIG. 13the same as or analogous to elements shown in FIG. 1 are referenced bythe same numerals.

With this embodiment, the maintenance apparatus further provided with amonitoring-item selection (MIS) unit 13 configured to select amonitoring item (i.e., an item to be monitored). Preferably, the MISunit 13 selects a monitoring item that can be monitored without haltingthe operation of the plant based on data on targets and ranges to bemaintained entered via the input unit 1. In this embodiment, the MISunit 13 select the vibration of a turbine shaft as the monitoring item,which is an intermediate, resultant failure event expected from thesummit event in the failure event tree.

If summit failure event is erosion of the steam-turbine nozzle blades,it is impossible to directly monitor the summit event, namely thecondition of the nozzle blades, while the plant is operating. Thus, inthis embodiment, vibration of the turbine shaft is monitored instead ofthe erosion of the nozzle blades. Abnormal vibration of the turbineshaft, due to change in steam flow caused by advancement of the nozzleerosion or the contact of the nozzle blades to the moving-blades, can bemonitored by using a vibration sensor, without halting the operation ofthe plant.

Monitoring item may be at least one of fluid pressure, temperature,rotation speed, load, vacuum, in-fluid impurity concentration, coolingspeed and time, deformation, displacement, decrease in wall thickness,fluid-level height and vibration, which can be monitored without haltingthe operation of the plant. In order to monitor the above items, themaintenance apparatus is provided with an on-line monitoring unit 14.The on-line monitoring unit 14 can utilize any sensors that detectelectric signals, ultrasonic wave or acoustic emission, for monitoringthe above factors while the plant is operating.

Shown in FIG. 14 is a failure-event tree for on-line vibrationmonitoring. FIG. 15 shows an interrelated matrix stored in thefailure-unreliability function database 8. Like the first and secondembodiments, the failure-event tree and data stored in thefailure-unreliability function database 8 are used for calculation ofrisk-based recovery and preventive maintenance expenses. The risk-basedrecovery cost and the preventive maintenance expenses are compared witheach other by an operation and maintenance decision-making (OMDM) unit15. Then, the OMDM unit 15 determines a time period for which anoperation at the time of maintenance decision can be continued and alsoinspection timing and preventive maintenance.

As disclosed above, the third embodiment achieves on-line determinationof real-time risk-based operation and maintenance while monitoringinspection-priority items in the failure-event tree without halting theplant.

Moreover, the third embodiment employs the on-line monitoring unit forremote abnormal-signal monitoring to at least one of several factors,such as, in-plant fluid pressure, temperature, rotation speed, load,vacuum, in-fluid impurity density, cooling speed and time, deformation,displacement, decrease in wall thickness, fluid-level height andvibration. The on-line monitoring unit can accurately monitor theabnormality of any items required for causal events or intermediateevents in the failure-event tree breakdown. Furthermore, the thirdembodiment employs any sensors using electric signals, ultrasonic wavesor acoustic emission, for monitoring those factors while the plant isoperating, thus accurately monitoring any signs of and the timing of thesummit event in the failure-event tree breakdown.

FIG. 16 shows a block diagram of the fourth embodiment of themaintenance apparatus according to the present invention. Elements shownin FIG. 16 the same as or analogous to elements shown in FIG. 1 arereferenced by the same numerals. The fourth embodiment employs aninformation network 16 connected to a communications network between theplant-maintenance apparatus according to the present invention andseveral plants B, C, D and E, and also a supervisor plant A that cantransmit data on unit-inspection, -operation and -monitoring and receiveresults of diagnosis and maintenance determination.

The signals received via the information network 16 are sent to areceived-information selection (RIS) unit 17 for selecting inputinformation among information for diagnosis and determination. Theselected information are stored in the failure-event tree database 7 andalso the failure-unreliability function database 8, for updating to dataon failure-event tree breakdown and unreliability. Failure-event treebreakdown and failure unreliability function calculation are performedbased on the updated data. After the same procedure as in the thirdembodiment, determined information on whether to continue plantoperations and inspection timing given at the OMDM unit 15 are returnedto the supervisor plant A.

As disclosed above, the fourth embodiment performs batch processing toinformation from several plants, thus can collect a lot of field data tobe used for decision of the failure-event tree breakdown and failureunreliability function, for further accurate diagnosis anddetermination. Moreover, the on-line monitoring unit offers informationon determinations and instructions based on real-time data, thuscontributing to steady plant operations.

As disclosed above in detail, the present invention offers quantitativerisk evaluation different from known experimental qualitative decisionmaking, thus achieving risk management covering any failure events toplant equipment and the minimum maintenance cost. Moreover, the presentinvention offers risk evaluation with the minimum number of inspectionitems or through on-line monitoring without a halt to plant operations,at the minimum inspection cost. Furthermore, the present inventionallows reference to information on several plants via network, thusachieving further accurate evaluation with extended field data.Therefore, the present invention achieves maintenance at minimum riskand cost.

1. An apparatus for maintaining plant equipment comprising: an input it configured to receive information on targets and ranges of plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; a failure-event tree breakdown unit configured to perform tree-breakdown on failures to be expected from a failure event to be inspected, in order to obtain a failure-event tree; a failure-unreliability function calculation unit configured to calculate unreliability of a resultant event in the failure-event tree with respect to a causal event in the failure-event tree preceding the resultant event; a failure-derived monetary loss calculation unit configured to multiply each unreliability and a cost of recovery from each event and accumulate results of multiplication in accordance with the failure-event tree breakdown, thus obtaining a recovery cost; a preventive-maintenance expense calculation unit configured to calculate preventive maintenance expenses for preventing failure events; and a maintenance decision-making unit configured to decide maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 2. The apparatus for maintaining plant equipment according to claim 1, wherein: the failure-event tree breakdown unit has a failure-event tree database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure-event tree breakdown unit is configured to perform the tree-breakdown from a specific failure-event of a specific part of the plant to be maintained inputted by an input unit, while referring to contents of the database.
 3. The apparatus for maintaining plant equipment according to claim 1, wherein: the failure-unreliability function calculation unit has a failure-unreliability function database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure unreliability function calculation unit is configured to calculate the unreliability while referring to the database based on failure-unreliability function between the resultant even and the causal event in the failure-event tree in accordance with information on parts of the plant and events inputted via the input unit.
 4. The apparatus for maintaining plant equipment according to claim 1, wherein the failure-event tree breakdown unit and the failure-unreliability function calculation unit adjust the order of the failure events in the failure-event tree and failure-unreliability function.
 5. The apparatus for maintaining plant equipment according to claim 1, wherein the unreliability of the plant equipment is given using cumulative hazard function based on an operation time at which each failure event occurs.
 6. An apparatus for maintaining plant equipment comprising: an input it configured to receive information on targets and ranges of plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; an inspection-priority item selection unit configured to select a failure event as a priority item to be inspected which is stored in a failure event tree database; an inspection unit configured to inspect the selected failure event; a failure-event tree breakdown unit configured to perform tree-breakdown on failures to be expected from the failure event to be inspected, in order to obtain a failure-event tree; a failure-unreliability function calculation unit configured to calculate unreliability of a resultant event in the failure-event tree with respect to a causal event in the failure-event tree preceding the resultant event; a failure-derived monetary loss calculation unit configured to multiply each unreliability and a cost of recovery from each resultant event and accumulate results of multiplication in accordance with the failure-event tree breakdown, thus obtaining a recovery cost; a preventive-maintenance expense calculation unit configured to calculate preventive maintenance expenses for preventing failure events; and a maintenance decision-making unit configured to decide maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 7. The apparatus for maintaining plant equipment according to claim 6, wherein: the failure-event tree breakdown unit has a failure-event tree database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure-event tree breakdown unit is configured to perform the tree-breakdown from a specific failure-event of a specific part of the plant to be maintained inputted by an input unit, while referring to contents of the database.
 8. The apparatus for maintaining plant equipment according to claim 6, wherein: the failure-unreliability function calculation unit has a failure-unreliability function database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure-unreliability function calculation unit is configured to calculate the unreliability while referring to the database based on failure-unreliability function between the resultant event and the causal event in the failure-event tree in accordance with information on part of the plant and events inputted via the input unit.
 9. The apparatus for maintaining plant equipment according to claim 6, wherein the failure-event tree breakdown unit and the failure-unreliability function calculation unit adjust the order of the failure events in the failure-event tree and failure-unreliability function.
 10. The apparatus for maintaining plant equipment according to claim 6, wherein the unreliability of the plant equipment is given using cumulative hazard function based on an operation time at which each failure event occurs.
 11. The apparatus for maintaining plant equipment according to claim 6, wherein the inspection unit comprises a CCD camera capable of accessing to a part to be inspected without disassembling a plant equipment in order to take a still image or a moving image, and a image analysis device configured to analyze a image data sent from the CCD camera.
 12. The apparatus for maintaining plant equipment according to claim 6, the inspection unit comprises an ultrasonic inspection device capable of detect a crack formed in a plant equipment from an outer surface thereof without disassembling the plant equipment.
 13. An apparatus for maintaining plant equipment comprising: an input unit configured to receive information on targets and ranges of plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; a monitoring-item selection unit configured to select a failure event as a priority item to be monitored which is stored in a failure event tree database; an on-line monitoring unit configured to monitor the selected failure event; a failure-event tree breakdown unit configured to perform tree-breakdown on failures to be expected from the failure event to be inspected, in order to obtain a failure-event tree; a failure-unreliability function calculation unit configured to calculate unreliability of a resultant event in the failure-event tree with respect to a causal event in the failure-event tree preceding the resultant event; a failure-derived monetary loss calculation unit configured to multiply each unreliability and a cost of recovery from each event and accumulate results of multiplication in accordance with the failure-event tree breakdown, thus obtaining a recovery cost; a preventive-maintenance expense calculation unit configured to calculate preventive maintenance expenses for preventing failure events; and a maintenance decision-making unit configured to decide maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 14. The apparatus for maintaining plant equipment according to claim 13, wherein: the failure-event tree breakdown unit has a failure-event tree database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure-event tree breakdown unit is configured to perform the tree-breakdown from a specific failure-event of a specific part of the plant to be maintained inputted by an input unit, while referring to contents of the database.
 15. The apparatus for maintaining plant equipment according to claim 13, wherein: the failure-reliability function calculation unit has a failure-unreliability function database constructed based on inspection histories and operation histories for the plant to be maintained and other plants; and the failure-reliability function calculation unit is configured to calculate the unreliability while referring to the database based on failure-unreliability function between the resultant event and the causal event in the failure-event tree in accordance with information on parts of the plant and events inputted via the input unit.
 16. The apparatus for maintaining plant equipment according to claim 13, wherein the failure-event tree breakdown unit and the failure-unreliability function calculation unit adjust the order of the failure events in the failure-event tree and failure-unreliability function.
 17. The apparatus for maintaining plant equipment according to claim 13, wherein the unreliability of the plant equipment is given using cumulative hazard function based on an operation time at which each failure event occurs.
 18. The apparatus for maintaining plant equipment according to claim 13, wherein the on-line monitoring unit is capable of remote abnormal-signal monitoring to at least one of factor that are in-plant fluid pressure, temperature, rotation speed, load, vacuum, in-fluid impurity density, cooling speed and time, deformation, displacement, decrease in wall thickness, fluid-level height and vibration.
 19. The apparatus for maintaining plant equipment according to claim 13, wherein the on-line monitoring unit utilizes electric signals, ultrasonic sound or acoustic emission, or their combination.
 20. A method of maintaining plant equipment comprising the steps of: receiving information on targets and ranges of plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; performing tree-breakdown on failures to be expected from a failure-event to be inspected, thereby obtaining a failure-event tree; calculating unreliability of every resultant event in the failure-event tree with respect to corresponding causal event in the failure event tree; multiplying each unreliability and a cost of recovery from each event to and accumulating results of multiplication in accordance with the failure-event tree breakdown, thereby obtaining a recovery cost; calculating preventive maintenance expenses for preventing failure events; and deciding maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 21. A method of maintaining plant equipment comprising the steps of: receiving information on plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; selecting failure event as a priority item to be inspected which is stored in a failure event tree database; inspecting the selected failure event; performing tree-breakdown on failures to be expected from the failure event to be inspected, thereby obtaining a failure-event tree; calculating unreliability of every resultant event in the failure-event tree with respect to corresponding causal event in the failure event tree; multiplying each unreliability and a cost of recovery from each resultant event and accumulating results of multiplication in accordance with the failure-event tree breakdown, thereby obtaining a recovery cost; calculating preventive maintenance expenses for preventing failure events; and deciding maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 22. A method of maintaining plant equipment comprising the steps of: receiving information on targets and ranges of plant equipment to be maintained, that constitute a plant, inspection information and operation-history information; selecting a failure event as a priority item to be inspected which is stored in a failure event tree database; monitoring the selected failure event; performing tree-breakdown on failures to be expected from the failure event to be inspected; calculating unreliability of each item in the failure-event tree breakdown; multiplying each unreliability and a cost of recovery from each resultant event and accumulating results of multiplication in accordance with the failure-event tree breakdown, thus obtaining a recovery cost; calculating preventive maintenance expenses for preventing failure events; and deciding maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expenses.
 23. A method of maintaining plant equipment comprising the steps of: transmitting, over a network, data from inspection and monitoring units for equipment that constitute each of a plurality of plants, each plant having a plurality of units; processing the transmitted data through failure-event tree breakdown, failure-unreliability function calculation, failure-derived monetary loss calculation, failure-derived monetary loss calculation and preventive-maintenance expense calculation; storing the processed data in a failure-event tree database and a failure-unreliability function database or learning, for providing the accuracy of failure-unreliability function calculation in maintenance decision making; and transmitting results of maintenance decision making to the plants. 